1. Field of the Invention
THIS INVENTION relates to the processing of metal alloys in a semi-solid state. In particular, the invention relates to a method of providing a contained charge of semi-solid metal alloy for use in a process for forming an article, to a rheo-casting container for containing a charge of semi-solid metal alloy, to a process for forming an article, and to an injection sleeve or shot sleeve for a die-casting machine.
2. Description of Related Art
The processing of metals or metal alloys in a semi-solid state is known as Semi-Solid Metals (SSM) technology. A known SSM processing route is that of thixo-casting. The thixo-casting processing route involves manufacturing billets having a desired microstructure (which is usually supplied to a forming facility by a producer or continuous caster) followed by re-heating to a semi-solid state and forming into the desired product. One of the known advantages of the thixo-casting process is that the forming facility is able to process the semi-liquid metal which readily lends itself to automation of the process. Some of the disadvantages of the thixo-casting process include the difficulty in obtaining fully homogenous billets in a continuous casting (electromagnetic stirred); metal losses during re-heating of the billet; and undesired oxidisation during the re-heating process on the surface of the billet. In addition, gates, runners and risers arising from the formed product cannot usually be re-cycled by the forming facility and must be sent back to the producer/continuous caster, which leads to additional costs.
Thixo-casting, in which the billets are moulded after they are heated to temperatures that produce semi-solid state metals, is different from another known processing route, namely, the rheo-casting processing route. In the rheo-casting processing route, molten metal alloy containing globular or spherical primary crystals is produced and moulded as such without being solidified into billets. In this process the liquid alloy is cooled down to a temperature between the alloy's liquidus and solidus temperature i.e. to provide an alloy in a semi-solid state. This is done in a controlled manner with agitation and, optionally, with the addition of grain refining agents, providing a slurry. The slurry is, subsequently, formed into the desired product. The object of the controlled cooling process and agitation is to avoid or impede dendritic crystallization and, instead, to promote the formation of globular or spherical primary crystals suspended in a liquid eutectic. The desired microstructure is obtained by the combination of controlled cooling, stirring and, optionally, the addition of a grain refining agent.
One of the advantages of the rheo-casting processing route is that the forming facility is able to re-cycle the scrap in-house and there are insignificant metal losses since there is no re-heating. One of the disadvantages with this processing route is that it includes a number of steps and the processes of which the Applicant is aware are thus cumbersome and complex, providing opportunities for optimisation and simplification.